Tyson Foods and Smithfield Foods are preparing to bring back online two of their biggest pork processing plants, which were shut down by coronavirus infections among workers.

Smithfield announced it would open up its plant in Sioux Falls, S.D., which had been the biggest source of COVID-19 cases in South Dakota. The reopening will take place in stages, with limited operations having already started, according to the unio representing plant workers. Full operation is scheduled to be phased in by late May.

Tyson, meanwhile, said it will reopen its plant in Waterloo, Iowa, which was closed in late April following more than 100 cases of COVID-19. The company said all employees returning to work had been tested for COVID-19, and that the plant had been deep-cleaned during the shutdown. The plant was scheduled to resume limited operations May 7.